The following is from delta.com:

Prohibited Ticketing Practices
Our fares apply only to the specific itineraries for which they are quoted and the restrictions that apply to our discounted fares are an essential part of our contract with you. These restrictions make it possible for us to offer these discounted fares.
Failure to comply with applicable fare restrictions, circumventing those restrictions, or misrepresenting your intended itinerary are all violations of our Contract of Carriage.
While not an exclusive list, the following ticketing practices are prohibited:
Back-to-back ticketing—combining multiple overlapping round-trip tickets to circumvent Saturday or other overnight stay requirements.
Throw-away ticketing—use of discounted round-trip excursion fares for one-way travel.
Point-beyond ticketing—use of a fare published for travel to a point beyond your actual intended destination or from a point before your actual intended origin.
In such cases where there is a violation of our Contract of Carriage, we reserve the right to:
Cancel the remainder of the itinerary and confiscate any unused flight coupons.
Refuse to board the passenger or check baggage.
Charge the passenger for the difference between the fare paid and the fare for the passenger's traveled itinerary.

As others have said, this would be back-to-back ticketing if the OP's proposed itineraries circumvent fare requirements.

It's probably best just to call Delta and ask. The OP makes a valid case for wanting to be on the same PNR with kids, especially in case of IROPs. That seems a reasonable justification for booking two itineraries.